This invention relates generally to apparatus for controlling a permanent magnet AC synchronous motor and more specifically the control of a permanent magnet AC synchronous motor supplied by a forced commutated inverter without the need of a mechanical shaft position sensor.
A system for controlling a permanent magnet AC synchronous motor must be adaptable to the fact that the motor field cannot be controlled and that when a force commutated inverter is used, motor operation may be in either the leading or lagging power factor made. A desirable mode of operation would be to operate with a small lagging to a small leading power factor for optimum motor efficiency. It would also be desirable to use an angle control which would eliminate the need of mechanical shaft position sensor such as the one described in U.S. Pat. No. 4,258,302, filed Sept. 28, 1979 and assigned to the General Electric Company, however, the above-mentioned angle control will operate only with leading or only with lagging power factors.
In an induction motor, the motor current always lags the voltage. Therefore, the electrical angle between the stator current and the air gap flux is always within the range of -90.degree. to +90.degree. with negative angles representing generating and positive angles representing motoring.
In a load commutated inverter supplying a synchronous motor the stator current must lead the voltage to maintain load commutation and therefore the motor cannot be operated in the lagging mode. The electrical angle between the stator current and the air gap flux is always greater than 90.degree. but less than 180.degree..
In a permanent magnet motor with a force commutated inverter, however, if the voltage supplied by the inverter is relatively large compared to the counter emf, then the electrical angle between the stator current and the air gap flux will be less than 90.degree.. If the inverter output voltage is lowered while maintaining the same torque output, the stator current can swing around in phase with the stator voltage and then to an electrical angle greater than 90.degree. measured relative to the air gap flux. If the sine of the electrical angle is used as an angle feedback signal in an angle control system, having .theta. go from below 90.degree. to above 90.degree. changes the slope of the sine function making it unsuitable for use in a feedback loop. Another difficulty with a permanent magnet motor is that at higher speeds the back emf of the motor will become greater than the supplied voltage and a leading power factor will result.
It is an object of the present invention to provide a permanent magnet synchronous motor control suitable for use with a forced commutated inverter that does not need a mechanical shaft position sensor and that can control operations from no load to maximum load while adaptively controlling the motor electrical angle so that nearly unity power factor operation can be achieved.